1. Field of the Invention
The present invention relates to apparatus for and a method of determining positional information for an object. The positional information might typically be angular orientation or attitude with respect to or range from a given datum.
The invention has application in a number of fields including but not limited to automotive collision avoidance, intruder detection, and sensors in robotics for supporting motion in a complex and changing environment. One particular use for the invention is in a scoring system of the type which can provide a score for the proximity of approach of an object such as a small aircraft or intruder to a specified location such as a target aircraft or other detection location.
The invention is described in detail later with reference to an electromagnetic sensor apparatus, and particularly with reference to short range electromagnetic sensing using short pulse, impulse systems (such as with pulses up to a few nanoseconds in duration). However, it could also function successfully using, for example, infra-red or acoustic techniques. A specific example of the present invention, which will be used to describe its preferred features, is that of a precision terminal trajectory scoring radar system.
2. Related Art
Electromagnetic sensors (impulse radars) employing short pulse transmissions have been used previously for evaluating the trajectories of various targets. In such applications, the requirement is often to measure the trajectory of the target in a region about the sensor, as well as target parameters such as the length or its orientation. For instance, an electromagnetic sensing apparatus is known from International Patent Application No. PCT/GB90/00602, which names Cambridge Consultants Ltd. as patent applicant now U.S. Pat. No. 5,181,039 issued Jan. 19, 1993 and whose disclosure is incorporated by reference.
FIG. 1 shows the configuration of this prior art apparatus, with an aircraft 10 notionally cocooned in a set of range gates 12. 16 range gates in total are shown. A target approaching the object (aircraft), as represented by the linear track 14 superposed on the range gates, will cross these range gates in a unique sequence determined by its relative trajectory. As can be seen from the linear track of the target, the different range gates are crossed at different times and so the solution for the full track is derived by assuming a particular form such as a straight line for the track.
FIG. 2 is a simplified schematic representation of this prior art apparatus showing just 2 of the 8 receive antennas typically provided. In brief, under the timing of timer 20 transmitter 22 transmits probe pulses via transmit antenna 14. The return signals are received via first and second receive antennas 26 and 28 and first and second RF amplifiers and filters 30 and 32. Under the timing of the timer 20 first and second sets of pulse generators PG1 to PG4 and PG5 to PG8 generate detection timing signals at varying delays with respect to the time of transmission of the probe pulse to trigger detection of the received signal at those delays by first and second sets of samplers S1 to S4 and S5 to S8. The various delays correspond to the various range gates 12 notionally surrounding the aircraft. Finally, the sampled signal is passed through first and second sets of audio frequency processing units AF1 to AF4 and AF5 to AF8, through first and second sets of data channels DC1 to DC4 and DC5 to DC8, to data processor 34 and data transmission means 36, and thence finally to a ground station 38 or the like.
In the present context, particular points of interest are that each range gate of the apparatus requires its own sampler, pulse generator and audio frequency (AF) processing unit and that the existence of independent pulse generators means that the differential delay between corresponding range gates with respect to different antennas is uncontrolled.
FIG. 3 shows a timing diagram for the prior art apparatus, for the sake of simplicity with just a single receiver and a single transmitter. Four range gates are formed using the four range gate samplers, at delays of 60, 122.5, 185 and 247.5 ns from the relevant probe pulse. In theory each sampler might be considered to yield not one but a sequence of range gates from each of the previous probe pulses, each spaced apart by 500 ns (equivalent to 76 m). However, in practice only a single range gate is formed because the signal from a target decreases as the fourth power of range and so, quite deliberately, any reflection from previous probe pulses is arranged to be below the noise floor of the apparatus and is hence undetectable by the samplers. Thus although the first sampler theoretically could give range gates at approximately 9, 85 and 161 m and so on, the signal at 85 m will be 39 dB below that from 9 m and so it will not be detectible; likewise the signals at 161 m and beyond will also be undetectable. Hence every sampled return signal corresponds unambiguously to one range gate.
Another feature of the prior art apparatus is that the timing module is fairly complex because each sampler for a receiver is driven with an independent clock which has to be set to the required delay between range gates.
A refinement of the prior art apparatus is known from Internal Patent Application No. PCT/GB94/00738, which also names Cambridge Consultants Ltd. as patent applicant and whose disclosure is also incorporated by reference. In this case, it was recognized that to achieve higher accuracy on smaller vehicles than that of the original apparatus would require very accurate knowledge of the location of the range gate and hence a real time autocalibration system was employed. This system while improving the performance had the disadvantages of requiring yet more circuitry for the autocalibration receiving means and timing generator, and of increasing the bandwidth of the signals required to evaluate target position.
In both of these prior art apparatuses, the three dimensional vector position of the target is evaluated by making absolute range measurements to specific features of the target to be tracked from receivers distributed around the vehicle on which the sensor is mounted. A key feature of the apparatuses is that the range measurements with respect to the different receivers are not simultaneous and so deriving the precise trajectory of the target requires fitting an assumed model trajectory (such as a straight line or a constant curvature curve) to these non-simultaneous measurements. Hence they have the disadvantage that until the full three dimensional trajectory of the target as a function of time has been computed, no representation of the three dimensional position of the target at a particular time is available; hence the processing is not real time. Also, there is critical reliance on the particular assumed model trajectory.
Yet a further feature of these two known apparatuses is that the accuracy of position measurements deteriorates for smaller vehicles carrying the apparatus, to the point where they become unusable for very small vehicles. This is because the known apparatuses measure the trajectory of the target with a precision determined by the minimum baseline of the antennas as installed, and thus the smaller the vehicle on which the system is mounted the less accurate the position measurement. This problem was addressed in PCT/GB94/00738 by incorporating an autocalibration system to measure in real time the absolute locations of the range gates with very high precision without affecting the normal operation of the sensor. This too had the practical limitations that the accuracy was limited if the antenna separation was less than 1 m and the extra hardware required was expensive.